The subject matter disclosed herein relates generally to diagnostic imaging systems, and more particularly to Positron Emission Tomography (PET) imaging systems and scatter correction for PET imaging systems.
PET imaging systems typically generate images depicting the distribution of positron-emitting nuclides in patients based on coincidence emission events detected using a detector system, usually configured as a ring assembly of detector blocks. The positron interacts with an electron in the body of the patient by annihilation, and then the electron-positron pair is converted into two photons. The photons are emitted in opposite directions along a line of response. The annihilation photons are detected by detectors on both sides of the line of response of the detector ring. The image is then generated based on the acquired emission data that includes the annihilation photon detection information.
Coincidence detection circuits connected to the detectors record only photons that are detected within a narrow time window by detectors located on opposite sides of a line joining the detectors to the point of annihilation. These detections are deemed to have occurred “simultaneously” and are termed coincidence events. The coincidence events indicate that the positron annihilations occurred along a line joining the two opposing detectors. The coincidence events detected by the PET detector ring assembly are typically stored within data structures called emission sinograms. An emission sinogram is a histogram of the detected coincidence events where each of a plurality of bins in the histogram represents a potential detector pair element.
Some gamma rays are deflected from an original direction due to interaction with a body part before reaching the detectors. Such events are termed scatter events. These scatter events, if used during image reconstruction, result in biased estimates of the activity distribution in the patient. Thus, a true activity distribution of radio-activity in the scanned body part of the patient does not result.
In order to correct for scattered coincidences in PET scanners, various scatter correction methods are known. Scatter corrections are generally performed in two steps, which are a single scatter correction step and a multiple scatter correction step. Model-based single scatter simulation methods are often used for the single scatter correction. For correction of multiple scatters, model-based simulation methods can be used, but are computationally intensive. Additionally, convolution methods may be used with a simple fixed kernel. However, these convolution methods can reduce the accuracy of the scatter estimation.